scripts/build: Add program header support for gen_relocate_app.py

Some linker scripts also associate memory regions with program headers
(PHDRS [1]). This patch adds support on gen_relocate_app.py to also
place a phdr alongside the memory region.

To keep things simple (and more natural), the format is basically the
same used in linker scripts - an space followed by ':<phdr_name>', like:

  SRAM2\ :phdr0:COPY:/home/xyz/zephyr/samples/hello_world/src/main.c

(Note the escape char before the space.)

[1] https://sourceware.org/binutils/docs/ld/PHDRS.html



Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>